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    <title>Document</title>
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    <div id="div1"></div>

    <script>
        //基本数据类型/简单数据类型
        //string  number  boolean  undefiend  null

        //补充: 

        //1.undefined
        //这个类型的值只有一个,就是undefined.
        //变量声明未赋值, 值就是undefined. 
        // let age ;
        // console.log(age);        //undefined
        // console.log(typeof age); //undefined


        //2.null
        //这个数据类型的值也是只有一个,就是null. 
        //获取元素获取不到,得到的就是一个null.
        //console.log(document.getElementById('div2')); //null
        //console.log(typeof null); //object
        //console.log(Object.prototype.toString.call(null)); //[object Null]



    </script>
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